Due to increased power generation by unsteady renewable sources like wind or solar existing gas turbine based power plants are increasingly used to balance power demand and to stabilize the grid. Thus improved operational flexibility is required. This implies that gas turbines are often operated at lower load than the base load design point, i.e. at lower combustor inlet and firing temperatures.
At the same time, emission limit values and overall emission permits are becoming more stringent, so that it is required to operate at lower emission values, keep low emissions also at part load operation and during transients, as these also count for cumulative emission limits.
State-of-the-art combustion systems are designed to cope with a certain variability in operating conditions, e.g. by adjusting the compressor inlet mass flow or controlling the fuel split among different burners, fuel stages or combustors. However, this is not sufficient to meet the new requirements.
To further reduce emissions and to increase operational flexibility sequential combustion has been suggested in DE 10312971 A1. Depending on the operating conditions, in particular on the hot gas temperature of a first combustion chamber it can be necessary to cool the hot gases before they are admitted to a second burner (also called sequential burner). This cooling can be advantageous to allow fuel injection and premixing of the injected fuel with the hot flue gases of the first combustor in the second burner.
Good cooling of the combustion chamber with low consumption of cooling gas is required for a safe and efficient operation of a gas turbine using such a sequential combustor arrangement. Conventional cooling methods either require heat exchanger structures which lead to high pressure drops in the main hot gas flow or suggest injection of a cooling medium from the side walls. Injection of a cooling gas reduces the main flow in the first combustor relative to the total mass flow which is detrimental to the efficiency of a gas turbine operated with such a sequential combustor arrangement and a controlled cooling of the whole flow is difficult.